Usually, the bit rate necessary to transmit an audio and/or video signal with sufficient quality is an important parameter in telecommunications. In order to reduce this parameter and then to increase the number of communications possible via one and the same network, audio encoders have been developed in particular to compress the quantity of information necessary to transmit a signal.
Certain encoders make it possible to achieve particularly high ratios of compression of the information. Such encoders usually use advanced techniques for modeling and quantizing the information. Therefore, such encoders transmit only models or partial data of the signal.
The decoded signal, although it is not identical to the original signal (since a portion of the information has not been transmitted because of the quantization operation), nevertheless remains very similar to the original signal. The difference, from the mathematical point of view, between the decoded signal and the original signal is then called “quantization noise”. It is also possible to speak of “distortion” introduced by encoding decoding.
The compression processes of signals are often designed so as to minimize the quantization noise and, in particular, to make this quantization noise as inaudible as possible when it involves processing an audio signal. There are therefore techniques taking account of the psycho-acoustic characteristics of hearing, for the purpose of “masking” this noise. However, to obtain the lowest possible bit rates, the noise may remain audible, on occasions, which, in certain circumstances, degrades the intelligibility of the signal.
In order to reduce this noise, two families of techniques are usually used.
It is possible, first of all, to use a perceptual postfilter, of the type used for example in the speech decoders of CELP (for “Coded Excited Linear Prediction”) type. This involves filtering which improves the subjective quality at the price of distortion. Specifically, an attenuation of the signal is applied in the zones in which the quantization noise is the most audible (particularly between the formants). Current perceptual postfilters provide good results for speech signals, but less good results for other types of signals (music signals, for example).
Specifically, a postfilter for improving encoded speech is described in particular in the document by Chen et al.:
“Adaptive Postfiltering for Quality Enhancement of Coded Speech”, Chen J. H., Gersho A., IEEE Trans. On Speech and Audio Proc., (January 1995).
The model described is based on a division into two sections:                a “long term” section reinforces the harmonics (harmonics of the fundamental frequency) and hollows out the spectral valleys between these harmonics, and        a “short term” section reinforces the formants and also hollows out the spectral valleys between these formants.        
Harmonics and formants are well known spectral characteristics of speech but to apply this type of process to a signal other than speech generates great distortions. For example, the spectral richness of a music signal cannot be processed with such a simple signal model.
Therefore, perceptual postfilters can generate distortions because they are based on a model which is not precise enough. Moreover, the perceptual postfilter is usually ineffective in periods of silence. These problems were able to be observed experimentally by the Applicant which initially sought to incorporate this type of perceptual postfilter in decoders that are not of the CELP type, for example in decoders within the meaning of the G.711 standard or the G.722 standard.
Another processing family aims at conventional noise-reduction processes in order to distinguish the effective signal from the spurious noise. This type of process therefore makes it possible to reduce the noise associated with the environment of signal capture and it is often used for speech signals. However, in this instance, it is impossible to make the process transparent with respect to the noise associated with the sound pick-up environment, which poses a problem for the encoding of a music signal in particular. Therefore, in encoding/decoding it is possible to want to transmit the ambient noise and it is then desirable that the noise reduction does not apply to this type of noise.
The present invention enhances the situation.